1. Field of the Invention
The present invention relates to a post or mast structure, which is particularly adapted to support a load above ground level, preferably an elongated load. This load may represent an object, which is supported by a plurality of mutually coacting posts, such as an overhead power line, or an elongated object which is supported solely by a single post and which projects outwardly on one side thereof, or which projects in balance from both side of the post.
2. Description of the Prior Art
Posts, or masts, are to be found in many different forms and for many different purposes, ranging from lattice-work mast structures for carrying 400 kV overhead power lines down to fencing posts of 50 mm in diameter. The posts may be grouted in the ground or simply secured by burying one end of the post in a pit or hole formed in the ground and by compacting natural stone around the post, so as to hold the post firmly. Flag posts and sign posts can be said to constitute particular examples of the posts referred to here.
The economic significance of a novel type of post depends upon the cost of and the type of post the novel post is intended to replace and the number of posts involved. In Sweden, more than eight million wooden posts are used today for supporting overhead power lines and telecommunication lines. By present day standards, an impregnated wooden post of this kind is estimated to have an active useful life of 40 years. There exist today overhead power line installations which are 50 years old and in which not a single post has needed to be replaced, although 40 years is the recognized useful life span of a wooden post. The mechanical strength of the post is calculated to be so impaired after this length of time as to render the post unsuitable and in need of replacement. It will be appreciated that the useful life span of such posts will be progressively shorter in the future, since the wood from which present day posts are produced and the wood from which posts have been recently produced is not of the same quality as that upon which present day standards have been based.
In addition to the scarcity in modern forests of rooted trees, which are suitable to be used for wood for posts for overhead power lines in excess of 10 kV, the impregnation of such now available wood has presented pronounced problems. The impregnating agent used hitherto, i.e. creosote tar, has been classified as toxic by the authorities. Consequently, anyone working with creosote impregnated posts must wear special protective clothing. Another drawback with creosote impregnated posts is that they may not be stored in the open air, due to the fact that the impregnation methods used result in moist posts, caused by incomplete absorption of the creosote tar and manifested in sticky wood surfaces.
Arsenic-copper salt solutions are alternative impregnating agents to creosote tar but, since these solutions have a shorter effective life span than creosote, they are not as economically viable. When considering the problems represented by the deterioration in the natural surroundings when facilities for impregnating wood are present which use such impregnating solutions, it is seen that the increased use of such solutions is counter-productive to the endeavor to provide improved environmental conditions.
Overhead power lines intended for more than 70 kV are supported by lattice-work posts or masts. In addition to being expensive to manufacture, such masts are highly unaesthetic and present an ugly feature in the surrounding landscapes. The need for power lines is increasing with the increasing need for electrical energy from progressively increasing production units to progressively higher consumer concentrations. In many areas or districts, this has resulted in multiple power cables or lines being erected in parallel. The posts or masts involved herewith detract greatly from the surrounding countryside and, in addition, present obstacles to agricultural machines working in the area. The same applies to posts used to carry telecommunication lines, although in this case the posts are not as high as the masts used to carry power lines and are not, therefore, as equally discernible to the eye.
Attempts, to reduce the extent to which such posts or masts encroach upon cultivated agricultural land, have resulted in power lines being run across land which is not used for agricultural purposes or across marshy territory. However, the erection of power of telecommunication line masts or posts in this latter territory is both difficult and laborious. Certain posts need to be anchored with the aid of dolphin-like shoring structures, and sometimes with the aid of some twenty or so auxiliary supportive posts.
Because of the limited flexibility of a wooden post, it is necessary to shore the post when a change in power or telecommunication line direction is effected, even though this directional change may be only moderate. The costs involved include the cost of the shores and tensioning devices required, e.g. bottle screws, and also the additional cost of the necessary concrete foundations or horizontal subsoil anchoring posts and the excavation work that needs to be undertaken in conjunction therewith.
The method used hitherto for erecting wooden posts for different purposes is one in which a pit is dug to a prescribed depth, in the case of posts for carrying 10 kV cables, a depth of 1.40 m, whereafter the root end of the post is placed in the pit and the post is lifted to a vertical position. The pit, or hole, is then fitted with available screened aggregate and the post is brought to a truly vertical position prior to filling in the pit and finally consolidating the packing material. The work of preparing post pits has been facilitated for many years by the use of earth drills and tractor carried vertical diggers. However, the ground surrounding the pits is often uneven or is inclined, which results at times in incomplete compaction of the aggregate intended to anchor the posts.
Another drawback with known wooden post support structures is that when two such posts are used to support a transformer, and even when four such posts are used for this purpose, and when one of the posts used becomes defective and must be changed, it is necessary to disconnect the transformer and lower it to ground level before the post can be changed. Subsequent to replacing the defective post, the transformer has to be lifted back into position and reconnected. Even though it is possible to plan the work involved, it necessitates an interruption in the power supply, which may be troublesome. As will be understood, it is necessary to restrict the future use of wooden posts, not only because of the aforementioned toxic risk presented by impregnated posts, but also because wooden posts are attacked by insects, or pests, other than those normally classified as infestants, or parasites, even though the posts have been thoroughly impregnated. It has been found in recent years that wooden posts are attacked by the black housefly (Campanatus liquiperda) and the red ant (Formica nufa), to an extent which is on a par with the damage caused by woodpeckers, fungi and mold. The latter cause mainly superficial damage, whereas the ants attack the core of the wood itself. The reason for this is probably because the core of the post is unable to absorb the impregnating agent used, since the wood resin is impregnable and impermeable to the impregnates used, and secondly because the natural habitats for ants have been greatly restricted by modern forestry. This, together with clear cutting of entire forests and subsequent ground preparation, has decimated all protective locations where ants may build their stacks. Ants, which live in stacks, and also horse flies to some extent, normally lay their eggs in tree stubs and dry furrows. When the ground is finally cleared and such stubs and furrows can no longer be found in the area, power line posts become the natural habitat of the ants.
The problems recited in the aforegoing with regard to cable or wire carrying posts apply with varying degrees to all types of wooden posts, irrespective of whether they are used to support cableways, so-called ski lifts, fences, road signs, advertising signs, or as flag poles.